These are long, thin filaments that protrude from the cell surface and enable bacteria to move.

• A. Pili
• B. Cilia
• C. Capsules
• D. Flagella

Answer: (D)

Flagella are the long, whip-like structures that drive bacterial movement by rotating like tiny propellers, propelling the cell through liquid and enabling swimming toward favorable conditions or away from dangers. Pili are shorter hairlike appendages mainly used for attachment and DNA transfer, not propulsion. Cilia are motile structures found in many eukaryotic cells and beat in coordinated waves rather than rotating to move bacteria. Capsules are protective layers surrounding some bacteria and aid in adhesion or immune evasion, not movement. So the filamentous structures that enable movement are flagella.